This invention relates to a power supply for subscriber terminal equipment in a carrier subscriber telephone system, and more particularly to a starter circuit for firmly establishing operation of electrical circuitry including a DC-to-DC converter and oscillator means prior to the starter circuit being disabled for reducing power dissipation in the power supply.
The power supply in the subscriber station terminal of a carrier subscriber telephone system may comprise a switched DC-to-DC converter providing an 8-volt DC supply voltage for powering an oscillator that keys a pulse width modulator for causing it to drive the converter, a 250 volt DC supply voltage that is chopped for producing a ringing signal, and a 16 volt DC supply voltage that powers a subscriber channel unit. The power supply is connected to the carrier line and powered by line current from central office battery equipment. A starter circuit is required to ensure that the power supply starts operating when it is first connected to the line. In order to provide maximum output power to the subscriber terminal equipment connected to the power supply, it is necessary to disable the starter circuit once the power supply is operating satisfactorily. It is also necessary to ensure that operation of the power supply is firmly established before the starter circuitry is turned off. In a simple starter circuit, both a voltage divider and the series combination of a Zener diode and the emitter-collector path of a transistor are electrically connected across the input lines with the divider tap voltage applied to the transistor base electrode for turning the transistor and Zener on to establish a collector voltage that initially powers the oscillator and modulator. An output of the oscillator is applied to the transistor base electrode for turning off the starter circuit. Such a starter circuit was found ineffective since it resulted in a spurious operation mode in which the starter turned on intermittently and operation of the rest of the power supply was never fully established. Prior art starter circuits are shown in U.S. Pat. Nos. 3,085,211, Apr. 9, 1963, Converter With Active Starter Circuit, by J. L. Jensen et al and U.S. Pat. No. 3,527,999, Sept. 8, 1970, Switching-Type Voltage Regulator Controlled by Integrating Circuit by A. D. Winberger.
An object of this invention is the provision of a starter circuit that is maintained operative for producing a constant DC output voltage until operation of external circuitry is firmly established.